Most blade stabilizers used in the drilling of petroleum wells are of fixed design, not subject to being controlled from the surface of the well. Stabilizers are incorporated in the drill string and serve to centralize the drill string in the well hole and to stabilize it against motions away from the well hole axis. The stabilizers are usually placed in the drill string at some depth below the surface, it being necessary to withdraw the drill string from the well hole to install or relocate the stabilizers. This invention seeks to provide stabilizer apparatus which may be run into a well as part of the drill string and expanded and/or retracted to perform its stabilizer function when desired at a later time, and which may be repeatedly expanded and retracted as often as may be desired.